Fuels for Advanced Nuclear Energy Systems
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D. Petti, D. Crawford, and N. Chauvin Abstract Fuels for advanced nuclear reactors differ from conventional light water reactor fuels and also vary widely because of the specific architectures and intended missions of the reactor systems proposed to deploy them. Functional requirements of all fuel designs for advanced nuclear energy systems include (1) retention of fission products and fuel nuclides, (2) dimensional stability, and (3) maintenance of a geometry that can be cooled. In all cases, anticipated fuel performance is the limiting factor in reactor system design, and cumulative effects of increased utilization and increased exposure to inservice environments degrade fuel performance. In this article, the current status of each fuel system is reviewed, and technical challenges confronting the implementation of each fuel in the context of the entire advanced reactor fuel cycle (fabrication, reactor performance, recycle) are discussed.
Introduction This article describes the fuels and their associated technical issues for very high temperature reactors (VHTRs); sodium fast reactors (SFRs);1 gas fast reactors (GFRs); and, to a lesser extent, lead fast reactors (LFRs).2,3 [There is not a solid fuel form in molten salt reactors (MSRs) because the fuel is dissolved in the salt. In supercritical-water-cooled reactors (SCWRs), traditional light water fuel rods are used.] Although these fuel designs are quite different, their basic functional requirements are the same, summarized as follows: 䊏 to retain hazardous radionuclides in all but the most unlikely postulated conditions, 䊏 to maintain a geometry that can be cooled, 䊏 to maintain fissionable material in a controlled and predictable geometry, and 䊏 to provide a convenient form for fuel handling. Other mission-specific or system-specific requirements include reliable operation at high temperatures; compatibility with post-irradiation disposal or recycling technology; and technology-specific requirements for physical properties such as actinide element density, thermal conductivity, and melting temperature. Non-
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quantitative examples of such requirements can be found in Reference 4. Neutron irradiation, high temperatures, and accumulation of fission products all work to degrade and stress the fuel’s ability to meet these requirements. These factors limit the in-service lifetime or utilization of the fuel. In general terms, the degradation mechanisms that operate in current fuel designs include: 䊏 chemical attack of the fuel cladding or fuel particle layers by fission products or fuel constituents, which weakens the barrier properties; 䊏 stress of the cladding or fuel particle layers caused by increasing fission gas pressure and/or by volumetric swelling of the fuel material due to accumulating gaseous and solid fission products retained in the fuel material; and 䊏 irradiation effects in the cladding or fuel particle layers, which can lead to embrittlement, enhanced creep damage, or dimensional changes. (Such dimensional changes can be caused by void swelling
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